Generalisation. Not a big turn on for many a lass.
THe power(of engine), the thrill, the leathers, the fact that you can put your arms round them legitimately and hold on for dear life - and not be thought clingy!
I believe it's the feeling of the wind in the face, the sound and the rhythm. When you put all 3 together it sets a therapeutic response to the body and brain. But, that's just my experience
